[Description of the Prior Art: FIG. 3]
A conventional automatic gain control device will be described with reference to FIG. 3. FIG. 3 is a block diagram of a conventional automatic gain control device.
As shown in FIG. 3, the conventional automatic gain control device includes a variable gain amplifier 21, an A/D converter 22, an average power detection circuit 23, and a gain control circuit 24.
The variable gain amplifier 21 amplifies an input received signal with a gain controlled by the gain control circuit 24 and outputs the amplified signal.
The A/D converter 22 converts an analog signal to a digital signal.
The average power detection circuit 23 outputs an average power value by detecting an average power of the received signal at a particular time.
The gain control circuit 24 compares the average power value detected by the average power detection circuit 23 to a predetermined threshold and controls the gain of the variable gain amplifier 21 based on the result.
The operation of the conventional automatic gain control device shown in FIG. 3 will be described.
The received signal is amplified with a gain corresponding to a signal level by the variable gain amplifier 21, and the amplified signal is converted into a digital signal by the A/D converter 22. Then, an average power value is calculated by the average power detection circuit 23.
Then, the gain control circuit 24 compares the detected average power value to the threshold and performs the control on the gain for the variable gain amplifier 21 based on the comparison result.
Thus, when controlling the gain based on the average power value, it is necessary to perform the gain control in expectation of peak power in order to prevent the overflow of the A/D converter 22.
For example, in a system using the adaptive modulation, the gain control is performed by a modulation scheme in which a peak to average power ratio (PAPR) is the highest.
Further, in a transmitter, a limiter technique for improving the power efficiency by clipping a peak signal is used and the PAPR varies even in the same modulation scheme. However, when considering the compatibility of the system, the gain control needs to be performed considering the transmitter with a high PAPR.
[Another Conventional Automatic Gain Control Device: FIG. 4]
Next, another conventional automatic gain control device will be described with reference to FIG. 4. FIG. 4 is a block diagram of another conventional automatic gain control device.
As shown in FIG. 4, another conventional automatic gain control device includes a variable gain amplifier 31, an A/D converter 32, a peak power detection circuit 33, and a gain control circuit 34.
The variable gain amplifier 31 and the A/D converter 32 are the same as those shown in FIG. 3.
The peak power detection circuit 33 detects an output of the A/D converter 32, detects a peak power value in a certain monitoring period (span), and outputs the detected value to the gain control circuit 34.
The gain control circuit 34 controls the gain of the variable gain amplifier 31 such that the peak power value is equal to a full scale (maximum input level which is not saturated) in the A/D converter 32.
The operation of another conventional automatic gain control device shown in FIG. 4 will be described.
The received signal is amplified with a gain corresponding to a signal level by the variable gain amplifier 31 and the amplified signal is converted into a digital signal by the A/D converter 32. The peak power value of a certain period is detected by the peak power detection circuit 33.
Then, the gain control circuit 34 controls the gain of the variable gain amplifier 31 such that the detected peak power value is equal to the full scale of the A/D converter 32.
Thus, when controlling the gain by detecting the peak of the signal level of the certain monitoring period, the dynamic range of the A/D converter can be effectively utilized, but it is hard to respond to a momentary increase in signal level, and overflow occurs if a large signal level is inputted instantaneously.
[Related Art]
As a technique relating to AGO (Automatic Gain Control), there is Japanese Patent Application Publication No. 2002-77101 (Hitachi Kokusai Electronic Inc., Patent Document 1).
Patent Document 1 discloses an OFDM (Orthogonal Frequency Division Multiplexing) signal receiver which calculates an average power of a received signal, compares the average power to a reference voltage, controls the gain of the AGC on the basis of the comparison result, and performs switching by selecting one of reference voltages on the basis of the length of the effective symbol period of the OFDM signal.
[Patent Document 1] Japanese Patent Application Publication No. 2002-77101
However, in the conventional automatic gain control device which controls the gain based on the average power, the PAPR of the received signal is expected in advance, and the gain is set to avoid the overflow of the A/D converter. Accordingly, if the PAPR of the received signal is smaller than the expected PAPR, the dynamic range of the A/D converter cannot be fully utilized and the influence of the noise of the A/D converter increases. On the contrary, if the PAPR of the received signal is greater than the expected PAPR, there is a problem in that the A/D converter overflows and the reception characteristics are degraded.
Further, in the conventional automatic gain control device which controls the gain based on the peak power, in order to control the gain according to the peak power that constantly changes, it is necessary to appropriately set the threshold or the control period. Therefore, it is quite difficult in practice, and there is a problem in that the overflow in the A/D converter occurs frequently, and the reception characteristics are degraded.